In the type of magnetometer-type compasses which incorporate three orthogonally disposed providing output signals corresponding to the two horizontal components, Hx and Hy, and a signal corresponding to the vertical magnetic field component, Hz, the compass heading .theta. is calculated according to the equation EQU .theta.=arc tan (Hy/Hx) (1)
and the Hz component is reserved for other positioned calculations. Such compasses typically dispose the horizontal field (Hx and Hy) sensors on a common planar structure and movable about both horizontal axes, and include a pendant weight disposed orthogonal to the plane having the Hx and Hy sensors so that the Hz component is directed radially inward towards the earth's center, and that the planar structure is disposed tangentially to the earth's surface. Such compasses are typically electronically and mechanically complex and are contained within a sealed, fluid-filled housing making accurate adjustment of the assembled elements difficult. When the horizontal field measuring components are not perfectly orthogonal to the Hz component measuring element, the heading as calculated by the equation (1) will in certain conditions, such as latitude extremes, include significant errors therein unless accurately recalibrated.
Compass calibrations have typically involved field calculations wherein the platform is moved to a variety of different locations at the earth's surface and the calculated heading corrected according to errors between the known locations and the heading produced by the compass. Such calibration procedures are clearly inconvenient and frequently inaccurate.